Kelim, Chapter Sixteen, Mishnah Seven
1) The dung bag of a bull and its muzzle, a bee shelf, and a fan are clean.
2) The cover of a small box is susceptible to uncleanness;
3) The cover of a clothes chest is clean.
4) The cover of a box, the cover of a basket, a carpenter’s vice, a cushion under a box or its arched cover, a reading-desk for a book, a bolt-socket, a lock-socket, a mezuzah case, a lyre case, a violin case, the block of the turban-makers, a wooden musical toy horse, the clappers of a wailing woman, a poor man’s parasol, bed posts, a tefillin mould, and the mould of string makers all these are clean.
5) This is the general rule which Rabbi Yose stated: all objects that serve as a protection to objects that a man uses, both when the latter are in use and when they are not in use, are susceptible to uncleanness; but those that serve them as a protection only when the latter are in use are clean.
Section one: The dung bag is attached to the bull and used to collect its dung when it is threshing. Think of it as a bull-diaper (I wouldn’t want to be one to change it!). The muzzle keeps the bull from eating. The bee shelf is placed in front of the hive and the bees sit on it before entering. None of these objects is susceptible because none of them are considered to be vessels.
Section two: Since people will use the cover of the small (jewelry) box to hold things, even temporarily, it is susceptible.
Section three: In contrast, people don’t use the cover of a clothes chest, so it is not considered a vessel.
Sections four-five: The mishnah now contains a long list of items that are clean. Rabbi Yose’s general rule in section five explains why they are pure. If an item is used to protect and store other objects then it is susceptible. The fact that it is used to store something when not in use proves that we consider it a container. However, if it is only used when the object it serves is in use then it is not considered to be susceptible. For instance, if a lid is only used on a container when the container is full, then the lid is pure. But if the lid is always on the vessel, then it is susceptible.
We should note that some of the objects in section four would seem to be used for storage. Therefore commentators explain that the “cases” here do not have receptacles and therefore are not susceptible to impurity.